1. Technical Field
This invention relates generally to a system and method for controlling linear motors and, more particularly, to an improved system and method that increases apparent stiffness for such motors while using a digital electronic drive.
2. Discussion of the Prior Art
In multi-axis positioning devices for heavy industrial cutting tools, linear motors can desirably replace mechanical (ball screw) drives actuated by rotary motors. Mechanical drives have inherent stiffness or resistance to external forces arising from the friction between parts of the drive. In linear motors, force is transferred directly by noncontact electromagnetic field variations and thus lacks inherent stiffness to resist external forces. However, through the use of a control system, such motors can be given apparent stiffness.
Attempts by the prior art to provide such apparent stiffness have been isolated to light load applications and have resulted in low stiffness ranges (10,000-50,000 pounds/inch). Such stiffnesses are well below those necessary to meet the quality control requirements of high-speed, heavy industrial machine tool applications. Such attempts have used accelerometers which have not proved helpful in conjunction with heavy duty digital drives because (i) they reject disturbances in only one axis of motion, (ii) require cross-coupling in multi-axis machines, thereby limiting the ability to fine tune the performance of the machine, and (iii) require additional hardware, adding to the complexity of the electrical servo-loop causing a slower response. Exemplary of such art is U.S. Pat. No. 4,808,901, showing a control apparatus for linear motors used to position an optical disc reading element. Dedicated chips process analog signals from a motor positioning detector to provide a "present" position, velocity, and acceleration signal; these signals are then compared to target signals and adjustments are made to the analog signal to send a new current signal to the motor. A significant drawback of such analog control system is the difficulty of critically setting up the control at precise parameters and changing the control to suit new needs or to debug the system. Altering resistance or capacitance of the chip elements is expensive and time-consuming.